Immersed membrane apparatus

ABSTRACT

An apparatus for filtering a liquid in a tank has a plurality of elements and a frame for holding the elements while they are immersed in the liquid. The elements have a plurality of hollow fibre membranes attached to and suspended between an upper header and a lower header. The membranes are in fluid communication with one or more permeate channels in one or more of the headers. Releasable attachments between the headers and the frame allow the frame to releasably hold the elements by their headers. The size and configuration of the frame determines the positions of the upper and lower headers of each element relative to each other. Connections between the permeate channels and one or more permeate collection tubes attached to the frame are releasable and resealable connections which are made or broken automatically by the movements involved in inserting or removing an element into or out of the frame.

This is a continuation of U.S. application Ser. No. 10/146,934 filed May17, 2002, which is a continuation-in-part of U.S. Ser. No. 09/889,352filed Jul. 17, 2001, issued as U.S. Pat. No. 6,790,360 on Sep. 14, 2004,which is a National Stage entry of PCT/CA00/01359 filed Nov. 15, 2000.All of the applications listed above are incorporated herein, in theirentirety, by this reference to them.

FIELD OF THE INVENTION

This invention relates to filtering membranes and particularly tomodules of immersed, suction driven, ultrafiltration or microfiltrationmembranes used to filter water or wastewater.

BACKGROUND OF THE INVENTION

Submerged membranes are used to treat liquids containing solids toproduce a filtered liquid lean in solids and an unfiltered retentaterich in solids. For example, submerged membranes are used to withdrawsubstantially clean water from wastewater and to withdraw potable waterfrom well water or surface water.

Immersed membranes are generally arranged in modules which comprise themembranes and headers attached to the membranes. The modules areimmersed in a tank of water containing solids. A transmembrane pressure(“TMP”) is applied across the membrane walls which causes filtered waterto permeate through the membrane walls. Solids are rejected by themembranes and remain in the tank water to be biologically or chemicallytreated or drained from the tank.

U.S. Pat. No. 5,639,373, issued to Zenon Environmental Inc. on Jun. 17,1997, describes one such module using hollow fibre membranes. In thismodule, hollow fibre membranes are held in fluid communication with apair of vertically spaced headers. TMP is provided by suction on thelumens of the fibres through the headers. Other modules are shown inU.S. Pat. No. 5,783,083 issued to Zenon Environmental Inc. on Jul. 21,1998, PCT Publication No. WO 98/28066 filed on Dec. 18, 1997 by MemtecAmerica Corporation and European Patent Application No. EP 0 931 582filed Aug. 22, 1997 by Mitsubishi Rayon Co., Ltd. As discussed in thesedocuments, various means are provided for fixing modules togethergenerally permanently into larger units.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve on the prior art. Itis another object of the present invention to provide a filtrationapparatus comprising a plurality of elements, for example, elements ofimmersed, suction driven, hollow fibre membranes, mounted to a frame.Embodiments of the invention provide few components to interfere withthe flow of tank water through the apparatus, efficient permeate pipeconnections, elements that may be removed easily and without interferingwith adjacent elements, elements that may be economically manufacturedto a wide range of sizes, an apparatus that may be assembled withvariable spacing between elements, and a distance between headers of theelements that can be altered to account for membrane shrinkage in use.The objects of the invention are met by the combinations of features,steps or both described in the claims. The following summary may notdescribe all necessary features of the invention which may reside in asub-combination of the following features or in a combination of some orall of the following features and features described in other parts ofthis document.

In various aspects of the invention, the invention is directed at anapparatus for filtering a liquid in a tank having a plurality ofelements, and a frame for holding the elements while they are immersedin the liquid. The elements have a plurality of hollow fibre membranesattached to and suspended between an upper header and a lower header.The membranes are in fluid communication with one or more permeatechannels in one or more of the headers. Releasable attachments betweenthe headers and the frame allow the frame to releasably hold theelements by their headers. While the frame is holding the elements, theelements themselves do not have any means for holding the headers inposition relative to each other. For example, if the frame were removed,the headers would be free to move out of position relative to eachother. As a result, the size and configuration of the frame determinesthe positions of the upper and lower headers of each element relative toeach other. When out of the frame, the elements may be inserted into aseparate carrying frame, if desired, for transport or handling.

An assembled filtration apparatus, which may be called a cassette, has aplurality of elements held such that the membranes are generallyvertical when immersed in the liquid in the tank. The headers may beelongated in shape and held in a generally horizontal orientation whenthe membranes are immersed in the tank. The frame holds the elements soas to provide a spacing between adjacent elements and allows tank waterto rise vertically through the frame and past the elements.

To assemble a filtration apparatus, the upper headers are slid into theframe, for example, through track and slider mechanism that may supportthe element whenever about one quarter of the length of the upper headeris inserted into the frame. The lower header may similarly slide intothe frame, for example through another track and slider mechanism. Or,while the element is supported by the upper header, the lower header maybe swung into position to attach to releasable supports which engagewith the ends of the lower header.

The frame holds or restrains the elements in place, but the restraintprovided by the frame may be released for a selected elementindividually. The selected element may be removed by reversing the stepsfor assembly without disassembling the remainder of the module.Connections between the permeate channels and one or more permeatecollection tubes attached to the frame are releasable and resealableconnections which are made or broken automatically by the movementsinvolved in inserting or removing an element into or out of the frame.

The frame may have cross bars located on uprights, the cross barsholding the elements. The vertical location of the cross bars may bechanged from time to time to maintain the membranes in a slightlyslackened condition although their length may decrease in use.

Aerators are mounted generally below the elements and supply scouringbubbles to the cassette and circulate tank water. The elements may benarrow, each element being a rectangular skein of hollow fibres havingan effective thickness of between 4 and 8 rows of hollow fibres. Theheaders, which may be extruded, may be thin to not greatly increase thewidth of the element. The attachments between the frame and the elementsare positioned to provide horizontal spaces between adjacent elements,preferably at least one third of the width of the headers measured inthe direction of the horizontal spacing, to promote penetration of thebubbles and tank water into the elements.

Elements may be placed back to back in pairs separated by permeatepipes. The connections between the permeate pipes and the elementsrelease when an element is pulled out of the cassette and reseal whenthe element is replaced in the cassette. Thus a single element can beremoved for maintenance without disconnecting other parts of thepermeate pipe network. A large permeate collector may be connected to asmall group of elements by a short local permeate pipe with a valve thatpermits the small group of elements to be isolated. Thus, while waitingfor repair, permeation can continue with the remaining elements. Thelarge permeate collector may be located above the water surface andconnect to an even larger collector which may be located on the edges ofa tank.

The headers may be made of an extrusion which may be cut to any desiredlength and capped with caps. The horizontal distance between the crossbars of the frame can be altered by changing the dimensions of the frameor the location of the cross bars relative to the frame. Longer orshorter cross bars can be used which hold fewer or more elements. Thevertical distance between cross bars can be altered by changing thedimensions of the frame or the location of the cross bars relative tothe frame. Accordingly, a cassette may be produced in a variety of sizesby altering the length of cut of one or more of the header extrusion,the cross bars or the frame members.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be describedwith reference to the following figures.

FIG. 1 is a somewhat schematic front elevation of a filtering element.

FIG. 2 is a somewhat schematic side elevation of the filtering elementof FIG. 1.

FIG. 3 is an isometric view of a header of an element of a firstembodiment.

FIG. 4 is an elevation of the end of 4 adjacent headers of FIG. 3.

FIG. 5 is an isometric view of a frame for a cassette with headersattached.

FIG. 6 is a close up of the top of FIG. 5.

FIG. 7 is a diagrammatic drawing of part of two elements placed back toback and connected to a permeate pipe.

FIG. 8A is a perspective view of a header and releasable attachment of asecond embodiment.

FIG. 8B is a side view of the header of FIG. 8A.

FIGS. 9 and 10 are assembled and exploded views of a component of thereleasable attachment of FIG. 8.

FIG. 11 is a perspective view of parts of a frame and releasableattachments of the second embodiment.

FIGS. 12, 13 and 14 are perspective, front and side views of a frame andparts of releasable attachments of the second embodiment.

FIG. 15A is a perspective view of an element of a third embodiment.

FIG. 15B is a close up view of part of FIG. 15A.

FIGS. 16A, 16B and 16C are schematic views of elements and permeateconnections of a third embodiment.

FIG. 17 is a perspective view of a header and parts of a releasableattachment of the third embodiment.

FIGS. 18A and 18B are side views of the releasable attachment between alower header and frame of the third embodiment.

DETAILED DESCRIPTION OF THE INVENTION A First Embodiment

The following paragraphs describe a first embodiment that is shown inFIGS. 1 to 7. Although the description below may at times refer tospecific figures, some components discussed may be shown only in othersof FIGS. 1 to 7.

FIGS. 1 and 2 show simplified front and side elevations respectively ofa filtering element 10. The element 10 has a plurality of hollow fibremembranes 12 in the form of a rectangular skein 14 suspended between anupper header 16 and a lower header 18. The rectangular skeins 14 may bebetween four and eight layers of membranes 12 deep (five layers beingshown in FIG. 2), optionally up to 12 layers deep, and are in the rangeof several tens of membranes 12 wide. The element 10 itself does notinclude any permanently attached means for holding the headers 16, 18 inposition relative to each other but the element 10 may be connected to acarrying frame if required for transport or handling. The lack of meansfor holding the headers 16, 18 in position relative to each otherimproves the flow of tank water about the element 10 and avoids apossible source of damage to the membranes 12.

The membranes 12 typically have an outside diameter between 0.4 mm and4.0 mm. The length of the membranes 12 is chosen to maximize flux for agiven cost according to relationships known in the art and is typicallybetween 400 mm and 1,800 mm. The membranes 12 have an average pore sizein the microfiltration or ultrafiltration range, preferably between0.003 microns and 10 microns and more preferably between 0.02 micronsand 1 micron.

The upper header 16 has a permeate channel 20 in fluid communicationwith the lumens of the membranes 12. The membranes 12 in FIGS. 1 and 2are sealed in the lower header 18, but the lower header 18 may also havea permeate channel in fluid communication with the lumens of themembranes 12 to permit permeation from both ends of the membranes 12.The membranes 12 are potted into the upper header 16 (and any otherpermeating header) such that the membranes 12 are all closely spacedapart from each other. Potting resin completely surrounds the outsidesof the end of each membrane 12 to provide a watertight seal so thatwater can only enter the permeate channel after first flowing though themembranes 12. Suitable potting resins include polyurethane, epoxy,rubberized epoxy and silicone resin. One or more resins may also be usedin combination to meet objectives of strength and providing a softinterface with the membranes 12 and avoiding cutting edges.

A potting method like that described in U.S. Pat. No. 5,639,373, whichis incorporated herein by this reference, may be used to pot layers ofmembranes 12. Other potting methods known in the art, include methodsthat produce non-layered or random arrangements of the membranes, mayalso be used. In particular, the methods described in Canadian PatentApplication No. 2,308,234, filed May 5, 2000 by Zenon EnvironmentalInc., and in U.S. application Ser. No. 09/847,338, filed on May 3, 2001by Rabie et al., both of which are incorporated herein by thisreference, may be used. The thickness of the assembled mass of membranes12 may be between 18 and 40 mm. Headers 16, 18 to accommodate suchmasses of membranes may be 40 to 50 mm wide, typically 40 mm. Thepotting densities may be between 10% and 40%. For example, an element 10may use membranes 12 as used in commercially available ZW 500 (TM)modules made by Zenon Environmental Inc. which have an outside diameterof about 2 mm, an un-potted length (meaning the unsupported length ofmembrane 12 between the upper header 16 and lower header 18) of 1,600 to1,900 mm, and a pore size of approximately 0.1 microns.

Referring to FIG. 3, the upper header 16 is shown. The lower header 18is the same would be mounted in an inverted position. The upper header16 includes a body 22 preferably extruded from a suitable plastic suchas PVC or ABS. The extrusion can be cut to a wide range of sizes asdesired. A back cap 24 is attached to the body 22 by gluing or welding.The body 22 includes a key 26 running the length of the top of the upperheader 16. The back cap 24 is shaped to extend the key 26. The key 26fits into slots in cross bars 30 of which only short sections are shown.The back cap 24 has an upper wing 32 and a lower wing 34. The back cap24 and the body 22 each have an upper channel 36 and a lower channel 38.A front cap is attached to the front of the body 22 but has been omittedfrom FIG. 3 to show the cross-section of the body 22. The front cap neednot have any wings 32, 34 but it does have channels 36, 38,

Referring to FIG. 4, four upper headers 16 are attached to a section ofcross bar 30 spaced to leave about 20 to 25 mm between adjacent upperheader 16. The lower headers 18 are similarly attached to anothercrossbar 30 but in an inverted position. The cross bar 30 can be cut toany desired length. To avoid the need to cut slots 28 into a long crossbar, the one piece cross bar 30 shown can be replaced with a standardextruded section, such as an inverted “C” channel, which supports anysuitable hanger containing a slot 28. In that case, the standardextrusion is cut to a desired length and an appropriate number ofhangers are attached or slid into it which allows the number of elements10 to be easily varied.

The upper headers 16 and their associated upper wings 32, lower wings34, upper channels 36 and lower channels 38 are all designated a, b, c,d to indicate which of those parts is associated with which upper header16. As shown, the upper wing 32 of a first upper header 16 engages theupper channel 36 of an adjacent upper header 16 and the lower wing 34 ofthe first upper header 16 engages the lower channel 38 of an adjacentupper header 16 on the other side. But, the upper wings 32 and lowerwings 34 do not interfere with each other in the direction of the lengthof the upper headers 16. Accordingly, each upper header 16 can be movedinto or out of its position in a direction parallel to the upper header16. Further, although the cross bar 30 provides support at only onepoint, a moving upper header 16 is supported and vertically positionedby its adjacent upper headers 16 aong its travel. This makes it mucheasier to insert or withdraw an element 10 despite the lack of (a) meanswithin the element 10 itself for maintaining separation between theheaders 16, 18 or (b) continuous frame channels paralleling the lengthof each header 16, 18 which would add many parts, add to the overallcost and manufacturing time, as well as interfere with bubbles and tankwater moving past the headers 16, 18. A releasable catch can beincorporated into the slot 28 and key 26 structure, typically at thefront only, to provide a releasable restraint in the direction of theheaders 16, 18.

Referring to FIGS. 5 and 6, a cassette 50 includes a frame 40 forholding several elements 10. The frame 40 includes top and bottom, frontand back cross bars 30, uprights 42 and struts 44 as shown. Threeelements 10 (with membranes 12 removed for clarity) are shown beingwithdrawn from the frame 40. Extra blank (ie. unpotted) headers 48 areoptionally included between the uprights 42 to provide support for thewings 32, 34 of the first element 10 on each side. An element 10 may becompletely withdrawn and then supported by hand or a single elementcarrying frame (not shown) may be placed against the frame 40. Theelement 10 is then slid into the carrying frame which may allow theelement 10 to be more easily worked with.

The length of the uprights 42 is chosen as appropriate for any desiredlength of membranes 12. The vertical distance between cross bars 30 ischosen so that the membranes 12 will be slightly slacked, their freelength being, for example, 0.1% to 2% more than the distance betweenproximal faces of the headers 16, 18. Particularly in wastewaterapplications where the tank water will be warm, ie. 30-50 C, themembranes 12 may shrink within the first few weeks or months ofoperation. To account for this shrinkage, the uprights 42 may beprovided with a series of mounting holes 46 which allow at least one setof the upper or lower cross bars 30 to be moved to maintain themembranes 12 in a slightly slackened position. Although not shown, asuitable aerator (designs are known in the art) may be mounted to theframe 40 or placed on a tank floor below the frame 40 to provide bubblesfrom below the cassette 50. The aerator is designed and positioned toencourage bubbles and tank water to flow upwards through the frame 40and past the elements 10, through the spaces between adjacent elements10 and between the membranes 12 within the elements 10.

To connect the headers 16, 18 to permeate pipes, the back of anypermeating headers 16, 18 are fitted with header permeate connections 52that can be released and resealed to a permeate pipe located behind theheaders 16, 18 and permit movement of the element 10 parallel to theheaders 16, 18. For example, FIGS. 5 and 6 show commercially availableclip on adapters sold under the trade mark UNI-SPRAY. These connectors52, however, require a clip to be released at the back of the element 10which is difficult to do if the elements 10 are placed back to back toshare common permeate pipes.

Referring to FIG. 7, pairs of cassettes 50 (partially shown, frames 40omitted, for example) are placed back to back with a local permeate pipe60 in between them. The frames 40 (not shown) of the two cassettes 50are tied together to maintain a fixed distance between them. The upperheaders 16 (and lower headers 18 if they are permeating) include malefittings 54 which releasably form a seal with a female fitting 56attached to the local permeate pipe 60. The seal is made by means ofO-rings 58 fitted into O-ring grooves 66 in the male fittings 54. Themale fittings 54 are thus connected to a local permeate pipe 60 whichmay service a small number of elements 10, ie. 2-6 elements 10. Thelocal permeate pipe 60 has an isolation valve 62, for example a ballvalve located above the water line, which permits the small group ofelements 10 to be isolated from the rest of the cassette 50. The localpermeate pipes 60 connect into a larger permeate collector 64 which maybe located at the level of even larger collector which may be located atthe edge of a tank. Thus, the necessary connections may be made simplyand without expensive flexible pipes. If the bottom headers 18 are alsopermeating, appropriate male fittings 54 are attached to the bottomheaders 18 at the level of female fittings 56 on or in communicationwith a local permeate pipe extension 68 which may be an extension of thelocal permeate pipe 60. If the bottom headers 18 are permeating headers,then the top headers may not be.

A Second Embodiment

The following paragraphs describe a second embodiment, parts of whichare shown in FIGS. 8 to 14. Although the description below may at timesrefer to specific figures, some components discussed may be shown onlyin others of FIGS. 8 to 14 or in figures discussed with otherembodiments. The second embodiment is similar to the first embodiment inmany respects. Aspects of the second embodiment that do not differsubstantially from the first embodiment may not be described in thefollowing paragraphs which will concentrate on the features of thesecond embodiment which differ from the first.

A second lower header 118 is shown in FIGS. 8A and 8B. A second upperheader 116 (not shown in this figure) is similar, but mounted in aninverted position. The second lower header 118 has a second key 126 onits lower surface that may be continuous like that of the second header18. Optionally, the second key 126 may be segmented, for example asshown in FIG. 8B, which helps prevent the second key 126 from stickingin the second slot 128, which will be described below.

The second lower header 118 does not have an upper channel 36 or a lowerchannel 38. A second back cap 124 of the second lower header 188 alsodoes not have an upper wing 32 or a lower wing 34, but rather is of asimilar section as the second body 122 of the second lower header. Asecond front cap 125 is fitted to the front of the second body and has apull tool fitting 180 adapted to allow a tool to pull on the secondlower header 118 for removal.

FIG. 8A also shows a track piece 182 located below the second lowerheader 118. A similar track piece 182 would be located above the secondupper header 116. The track piece 182 provides part of a continuoussecond slot 128 that the second keys 126 may slide into and be supportedby. The track piece 182 is supported at both ends by the cross bars 30.For example, in the embodiment shown, one end of the track piece 182fits over and is supported by an abutment 184 attached to the side of apermeate pipe stub 186 resting on a cross bar 30. The permeate pipe stub186 is sealed at its lower end and ready to be connected to a localpermeate pipe extension 68 (not shown in FIG. 8A, refer to FIG. 7) atits upper end. The permeate pipe stub 186 also has female fittings 56 influid communication with the inside of the permeate pipe stub 186. Thefemale fittings 56 are located and oriented so that when the secondlower header 118 is fully inserted in the second slot 128, a malefitting 54 (not visible) is sealingly connected to the female fitting56. The other end of the track piece 182 in the embodiment shown issupported by a locking clip 188 which both supports the track piece 182relative to the cross bar 30, but also completes the second slot 128 andreleasably locks the second lower header 118 in position when the secondlower header 118 is fully inserted in the second slot 128. The lockingclip 188 is held in place by fitting into a cross bar channel 192 and islocated along the length of the cross bar 30 by interaction with apositioning hole 190. The description above also applies, but withinverted orientation, for the second upper headers 116.

FIGS. 9 and 10 show the locking clip 188 in greater detail. A lockingclip abutment 194 is sized and shaped to fit into and support the trackpiece 182. A peg 196 fits into a peg slot 198 to provide a means forlocating the locking clip 188 over a positioning hole 190. A catch 200fits over the body of the locking clip 188 and, in an unbent position,fills a part of the second slot 128. However, the catch 200 has taperedfaces so that the catch 200 can move out of the second slot when asecond key 126 is slid into the second slot 128. After the end of asecond key 126, or series of discontinuous second keys 126 passes thecatch 200, they are prevented from moving back out of the second slot126. However, a release hole 202 also provides access to the taperedfaces of the catch 200. By inserting a rod into the release hole 202,the catch 200 can be held open to allow a second key 126 to be pulledback out of the second slot 128. The locking clip 188 also has a foot204 sized to engage with the cross bar channel 192.

FIG. 11 shows how the bottom part of a frame 40 ready to receive secondelements 110. As shown, a pair of struts 44 are attached to a centralcross bar 30 a and two end cross bars 30 b, only one visible. Fourbrackets 206 are provided to attach to uprights 42. The central crossbar 30 a supports a number of permeate pipe stubs 186 which in turn haveabutments 184 holding track pieces 182. The end cross bars 30 b supportlocking clips 188 which support the other ends of the track pieces 182.If the second lower headers 118 were not permeating headers, then thecentral cross bar 30 a would also be used to support locking clips 188.

FIGS. 12 to 14 show a more fully assembled frame 40 forming part of asecond cassette 150. A second assembly like that shown in FIG. 11 isinverted and placed over the assembly of FIG. 11. Uprights 42 hold thetwo assemblies together. The connection between the uprights 42 and oneor both of the assemblies may be made though slots 208 which allow thedistance between the two assemblies to be adjusted to fit the secondelements 110. The distance between the two assemblies may also beadjusted after the membranes 12 have been used, for example, to accountfor shrinking. The upper track pieces 182 are held at the central crossbar 30 a by flow through permeate stubs 210 which connect the localpermeate pipe extensions 68 to local permeate pipes 60 which are in turnconnected to a permeate collector 64 mounted to upper mounting tabs 212at the top of the frame. Lower mounting tabs 214 at the bottom of theframe 40 may be used to mount an aerator grid below the second cassette150. Only one of various components, such as second elements 110, localpermeate pipe extensions 68, local permeate pipes 60 and isolationvalves 62, are shown for clarity, but these components would be repeatedacross the second cassette 150. Also, although the second cassette 150is shown as configured to collect permeate from second upper headers 116and second lower headers 118, it may be adapted for use with permeatingsecond upper headers 116 only by replacing the permeate pipe stubs 186shown on the lower central cross bar 30 a with locking clips 188 andreplacing the flow through permeate stubs 210 shown at the upper centralcross bar 30 b with permeate pipe stubs 186. For use with permeatingsecond lower headers 118 only, the female fittings 56 of the flowthrough permeate stubs 210 are plugged up or altered flow throughpermeate stubs not having female fittings 56 are provided.

A Third Embodiment

The following paragraphs describe a third embodiment, parts of which areshown in FIGS. 15 to 18. Although the description below may at timesrefer to specific figures, some components discussed may be shown onlyin others of FIGS. 15 to 19 or in figures discussed with otherembodiments. The third embodiment is similar to the first and secondembodiments in many respects. Aspects of the third embodiment that donot differ substantially from the first or second embodiment may not bedescribed in the following paragraphs which will concentrate on thefeatures of the third embodiment which differ from the first or second.

FIGS. 15A and 15B show a third element 310. The third element 310 has athird lower header 318 and a third upper header 316 which are similar tothe second lower header 118 and second upper header 116. However, thethird headers 316, 318 differ, for example, in having third keys 326,third back caps 324 and third front caps 325 unlike related componentsof the second headers 116, 118. The third element 310 shown has twopermeating third headers 316, 318, but like previous elements may bemade with either the third lower header 318 not a permeating header orthe third upper header 316 not a permeating header.

Referring to FIGS. 15B and 16A,B,C, the third headers 316, 318 havethird back end caps 324 with male fittings 54 that are offset from thecenter of the third back end caps 324. Third back end caps 324A (shownschematically in FIGS. 16A,B,C as having truncated tops) have a malefittings 54 offset to one side of the center while third back end caps324B (shown schematically in FIG. 16A,B,C as having rounded tops) have amale fitting 54 offset to the other side of the center. By placing oneof third back end cap 324A and one third back end cap 324B on the thirdheaders 316, 318, third elements 310I and 310II can be made have malefittings 54 offset to opposite sides. If both of the third headers 316,318 are permeating, then separate third elements 310I and 310II need notbe made, as one will be an inverted version of the other. In conjunctionwith third permeate pipe stubs 386 having female fittings 56 on eitherside, a variable horizontal spacing between third elements 310I and310II can be achieved with a single design of third permeate pipe stub386 and without needing a third permeate pipe stub 386 for each thirdelement 310. In particular, as shown in FIGS. 16A and 16C, swappingthird element 310I for third element 310II (or turning each thirdelement 310I or 310II over if both third headers 316, 318 arepermeating) significantly alters the space between third elements 310Iand 310II. By using third permeate pipe stubs 386 that can be mounted atvarious positions along a cross bar 30, two different spacings of all ofthe third elements 310 of a third cassette 350 can be achieved withoutrequiring a separate local permeate pipe 60 and local permeate pipeextension 68 for each third element 310 and with only one smallcomponent, the third back caps 324, manufactured in two versions. Theability to have variable spacing is useful, for example, because a widerspacing can be chosen for wastewater applications and a narrower spacingchosen for drinking water filtration. As shown in FIG. 16B, anintermediate spacing may also be achieved by using a pair of thirdelements 310II. The same intermediate spacing may also be achieved byusing a pair of third elements 310I.

The comments made in the paragraph above regarding the third permeatepipe stubs 386 similarly apply to third flow through permeate stubs 310.The third permeate stubs 310 also have a pair of mounting pins 220 oneach side to support the end of the third track piece 382 (to bedescribed below) at either spacing. Similar pairs of mounting pins 220may also be provided on the third permeate pipe stubs 386 if they willalso be used to support the ends of third track pieces 383, althoughthis is optional as will be described further below.

FIGS. 15B and 17 show a third key 326 that which has a key ridge 224.Although FIGS. 15B and 17 show only a third upper header 316, the thirdlower header 318 is the same, but is mounted in an inverted orientation.Similarly, other components of FIG. 17 may all be used in invertedorientation at the bottom of a third cassette 350. The key ridge 324provides a line of contact between the distal surface of the third key326 and the third track piece 382. Similarly, the edges of the thirdtrack piece 382 curl inwards to provide a line of contact with theproximal surfaces of the third key 326. These lines of contact are lessprone to fouling than planes of contact.

FIG. 17 also shows the connection between one end of the third trackpieces 382 and a cross bar 30. The connection between the other end ofthe third track piece 382 and the third permeate pipe stubs 386, orthird flow through permeate stubs 310, was discussed above. The endshown in FIGS. 15B and 17 is supported on a pin (not visible) one atrack mounting plate 226 mounted on a cross bar 30. The track mountingplate 226 also supports a third lock 388 that assists in keeping thethird track piece 382 in position. The third lock 388 also mates with atwist knob 228 to allow the third upper header 316 to be releasablysecured when it has been fully inserted into the third slot 328.

As an alternative to using an inverted version of the components shownin FIG. 17 to releasably attach the third lower header 318, FIGS. 18Aand 18B show how the third lower header 318 may be Releasably attachedto the frame 40 without using a third track piece 382. Referring to FIG.18A, the third top header 316 (not shown) is partially inserted, forexample between about one half to three quarters of the way, into thethird track piece 382 (not shown). At this point, the third lower header318 hangs from the membranes 12. The third lower header 318 is thenpushed into its final position which is shown in FIG. 18B. Because thethird upper header 316 was only partially inserted, the third lowerheader 318 arcs upwards slightly. Through trial and error or measurementand calculation, a position of the third upper header 316 can bedetermined at which the upward movement of the third lower header 318,despite the excess length of the membranes 12 required to produceslackened membranes 12 when the third element 310 is fully installed,allows the male fitting 54 to meet the female fitting 56 and allows thetwist knob 228 to meet and be releasably connected to the third lock388. The third upper header 318 is then fully inserted which restoresthe slack in the membranes 12. The twist knob 228 of the third upperheader 316 is then engaged with the third lock 388 of the upper part ofthe frame 40.

The embodiments described above are examples of the invention only.Modifications and other embodiments within the scope of the inventionwill be apparent to those skilled in the art. The scope of the inventionis defined by the following claims.

1. An apparatus for filtering a liquid in a tank comprising, a) aplurality of elements, each having, i) an upper header; ii) a lowerheader; iii) a plurality of hollow fibre membranes attached to andsuspended between the headers, the membranes having each at least oneopen end and an outer surface, the outer surface of the open ends of themembranes connected to at least one header with a water impermeableconnection; and, iv) one or more permeate channels in at least one ofthe headers in fluid communication with the interior of the hollow fibremembranes for collecting permeate; b) a frame for holding the pluralityof elements while the membranes are immersed in the liquid in the tank;wherein the size and configuration of the frame determines the positionof the upper header of each element relative to the lower header of eachelement.
 2. The apparatus of claim 1 wherein (d) the frame holds theplurality of elements in a position such that the membranes aregenerally vertical when immersed in the liquid in the tank, (e) theframe allows tank water to rise generally vertically through the frameand past the elements.
 3. The apparatus of claim 2 wherein thereleasable attachments include a track and slider mechanism between theframe and the upper headers to allow the upper headers to be slid fromthe frame.
 4. The apparatus of claim 3 wherein the track and slidermechanism is adapted to support the element whenever at least about onequarter of the upper header is inserted into the frame.
 5. The apparatusof claim 1 wherein the headers are elongated in shape and the frameholds the headers in a generally horizontal orientation when themembranes are immersed in the tank.
 6. The apparatus of claim 1 whereinthe releasable attachments between at least one end of the headershaving a permeate channel and the frame are made through one or morepermeate collection tubes.
 7. The apparatus of claim 6 wherein theelements are attached to the frame in a back to back configurationseparated by permeate collection tubes connected to the one or morepermeate channels of the elements.
 8. The apparatus of claim 1 whereinconnections between the permeate channels and one or more permeatecollection tubes attached to the frame are releasable and resealableconnections which are made or broken automatically by the movementsinvolved in attaching an element to the frame or removing an elementfrom the frame.
 9. The apparatus of claim 8 wherein pairs of headers areattached on either side of permeate collection tubes and fittings incommunication with the permeate channels on one or more of the headersare offset from a centerline of the headers such that the pairs ofheaders may be installed at different horizontal spacings from eachother.
 10. The apparatus of claim 3 wherein each element has anassociated releasable and resealable water tight fitting between theelement and a permeate collector, the releasable and resealable watertight fitting being releasable by moving the element in a directionsubstantially parallel to the headers of the element and resealable bymoving the element in a reverse direction.
 11. The apparatus of claim 3wherein the releasable attachments include a track and slider mechanismbetween the frame and the lower headers to allow the lower headers to beslid from the frame.
 12. The apparatus of claim 3 wherein the releasableattachments includes releasable supports on the frame which engage withthe ends of the lower header when the lower header is swung intoposition while the upper header is supported in the track and slidermechanism.